In the circuit above, the ammeter reads a current of 5.0 A when R =…

In the circuit above, the ammeter reads a current of 5.0 A when R = 8Ω and reads 7.0A when R = 5Ω. The value of the unknown resistance X is
  • A.
    10.0Ω
  • B.
    7.5Ω
  • C.
    5.0Ω
  • D.
    2.5Ω
Correct Answer: Option D
Explanation

From Ohms Law, E = I(R +X)
=> E = I1(R1 + X)
again, E = I2(R2 + X)
=> I1(R1 + X) = I2(R2 + X)
∴ 5(8 + X) = 7(5 + X)
40 + 5X = 35 + 7X
7X – 5X = 40 – 35
2X = 5
X = 5/2
= 2.5Ω

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